The present invention relates to fastener inserts or anchors and more particularly, to a sheet metal insert distinctly suited for use in structural foam plastic and other relatively soft material anchorage applications. The sheet metal insert is internally threaded and is adapted for insertion in a preformed hole in a workpiece to receive a standard type screw with an externally threaded shank. The respective threads of the insert and shank engage each other for the purpose of securing a part or other member to the workpiece of relatively soft material in which the sheet metal insert is fitted.
In the present age of advanced technology, there is an increasing requirement for usage of strong, lightweight materials in structural workpiece applications. One such material is structural foam plastic which can be manufactured from any suitable thermoplastic to which a foaming agent has been added, thereby increasing its volume by a substantial percentage. The structural foam plastic produced from this process often has a dense skin of approximately 0.010 to 0.030 of an inch in thickness. The material of the structural foam plastic becomes less dense as the center of the workpiece is reached.
Prior known types of fasteners or anchorage devices possess certain disadvantages when utilized with advanced relatively soft structural materials. A resultant compressive load is applied to the plastic workpiece when a screw or fastener is tightened or clamped. This compressive load causes the plastic material of the workpiece to creep in a relatively short period of time, thus substantially decreasing the clamp load and torque applied by the screw or fastener. This creeping action causes the fit between the workpiece and the part or member fastened thereto to become loose, structurally unsound, and thus wholly unsatisfactory.
There have been various known fastening or anchorage proposals that have incorporated sleeves or inserts with portions that could be displaced outwardly from within a preformed hole to dig or bite into the surrounding material, such portions being generally in the form of equal sized teeth or other projections. The disadvantage of such prior proposals is that it has been found that the resistance to axial pull-out or loosening provided by the teeth or other projections alone engaging the material is often not sufficient. This disadvantage is greatly magnified when the sleeves or inserts are utilized in soft materials such as structural foam plastic, where the density of the material decreases proportionally towards the center of the workpiece.
Many known sleeves or inserts also are constructed of relatively expensive materials, are complicated to make, incorporate several pieces, and hence are expensive to produce.
It is, therefore, desirable to provide an insert for anchoring a screw or fastener in relatively soft material that is specifically designed to prevent creep in the workpiece material when a compressive load is applied. It is additionally desirable to provide an insert that is specifically designed to maintain satisfactory resistance to axial pull-out or loosening when utilized in structural foam plastic type workpiece applications where the density of the material decreases proportionally towards the center of the material. It is moreover desirable to provide an insert which can be simply, efficiently, and economically produced, as well as installed using conventional methods.
The sheet metal insert of the present invention can be produced in a stamping process from a one-piece blank. The stamped piece can then be formed to its final configuration as a substantially circular insert.
The sheet metal insert includes a cylindrical body portion with a collar at one end that rests on the surface of the foam plastic workpiece and provides a first clamping face for joining a member to the workpiece. A first and second leg extend downward from the other end of the body portion, each leg having a series of inwardly and diagonally projecting indents or ribs that serve as the internal threads of the insert. On each side of the first and second legs, a series of protrusions or tangs project outwardly to form an angle with the legs. The sheet metal insert is designed so that each successive tang away from the collar projects further outward from the leg while maintaining the same angle with the leg. These progressively longer tangs compensate for the less dense material towards the center of the structural foam workpiece. The first and second legs are also deformed inwardly towards the axis of the insert so that the progressively longer tangs do not interfere with installation of the insert in the preformed hole of the workpiece.
When the insert is put into a cored or drilled hole, and a screw or other externally threaded stem is driven into the insert, the end of the screw, upon coming into engagement with the deformed legs, will deflect or push the legs outwardly, causing the tangs to bite into the plastic or foam material of the workpiece surrounding the hole.
The head of the screw defines a second clamping face that coacts with the first clamping face of the insert collar to clampingly engage and load opposite faces of the member desired to be fastened to the workpiece. The sheet metal insert of the present invention is designed so that the clamp load is taken between the internal thread ribs and the collar of the insert. In this manner, there is no resultant compressive or tensile force applied to the structural foam workpiece by the insert and hence creep will therefore not occur.
The above and other features of the invention will become apparent from a reading of the detailed description of the preferred embodiments, which makes reference to the following set of drawings.